From law 12. IFAB.
Handling the ball
For the purposes of determining handball offences, the upper boundary of the arm is in line with the bottom of the armpit. Not every touch of a player’s hand/arm with the ball is an offence.
It is an offence if a player:
deliberately touches the ball with their hand/arm, for example moving the hand/arm towards the ball
touches the ball with their hand/arm when it has made their body unnaturally bigger. A player is considered to have made their body unnaturally bigger when the position of their hand/arm is not a consequence of, or justifiable by, the player’s body movement for that specific situation. By having their hand/arm in such a position, the player takes a risk of their hand/arm being hit by the ball and being penalised
scores in the opponents’ goal:
directly from their hand/arm, even if accidental, including by the goalkeeper
immediately after the ball has touched their hand/arm, even if accidental